The present invention relates generally to a circuit for detecting unbalanced currents in a normally balanced multi-phase load, such as a three-phase motor.
In aircraft applications there are power system architectures that employ three-phase circuits. Some of these applications have three phase loads that are inherently balanced loads. For example, on equipment such as a motor drive, the load is a balanced three-phase load and the source voltage driving this load is also a balanced three-phase source. It is desired, for overall system power distribution protection, to detect unbalanced currents in a normally balanced three phase load.
For normal motor speed and torque control functions, the associated motor drive incorporates current sensing on the motor power line. Thus, the motor currents are already available for this unbalanced phase current detection function. However, because the current level provided to the lower is variable (but balanced) it is necessary to determine the existence of unbalanced current over a wide range of possible current levels. Also, because the motor operates over a wide range of speeds, it is necessary to determine the existence of unbalanced currents over a wide fundamental frequency range.